Poly(ethylene terephthalate) copolymers, commonly referred to as PET polymers, are widely used in the manufacture of light weight containers for carbonated and non-carbonated drinks, juice, water, jellies, marmalades, and other similar foodstuffs. Packages made by stretch blow molding of PET polymers possess excellent mechanical properties, such as high strength and shatter resistance, as well as good gas barrier properties.
Typically, to form plastic containers, the PET polymer is extruded and formed into chips or pellets. The pellets are then melted and used to make a container preform by injection molding. The preform is subsequently reheated and stretched-blown into a mold, which provides the final shape of the container. The stretch blow molding step causes biaxial orientation of the polyester to occur at least in some parts of the container and provides strength to the container so that it can resist deformation from internal pressure during use and can adequately contain the fluid.
There are three key characteristics of PET polymers that are relevant to making containers by stretch blow molding, namely, their natural stretch ratio, their crystallization rate, and the rate at which they fill injection molds.
The natural stretch ratio is an inherent property of a polymer and is a measure of how much the preform can stretch to take the shape of the final article. In the present Examples, the free blow volume of a given polymer is used as a measure of the natural stretch ratio of that polymer. The natural stretch ratio of a polymer influences the design of the preform by determining its stretch ratio limitations. Due to the high cost of injection mold tooling, new PET resins that perform well with existing preform designs can be used. Resins with very low natural stretch ratios generally create processing problems during stretch blow molding, while resins with very high natural stretch ratios generally yield containers with poor physical properties if used in conjunction with preform and bottle tooling.
The rate of crystallization of a PET polymer can influence the clarity or transparency of the final article. Therefore, controlling the rate of crystallization becomes relevant especially when the application requires clear or transparent products. Thermally induced crystallization tends to form large crystallites in the polymer, resulting in haze. In order to minimize the formation of crystallites and thus have clear preforms, the rate of thermal crystallization needs to be slow enough so that preforms with little or no crystallinity can be produced. However, if the rate of thermal crystallization is too low, the production rates of PET resin using solid state polymerization can be adversely affected because PET needs to be crystallized prior to solid-state polymerization.
The rate at which a polymer fills an injection mold is directly related to its intrinsic viscosity. A lower viscosity resin is generally desired because it will fill the injection mold more easily, leading to a reduction in injection molding cycle time and an increase in product output. Also, a lower viscosity resin will reduce the injection pressure required to fill the mold in a given time, reducing the wear and tear on the injection molding machine. Therefore, manufacturing costs may be reduced for resins with a lower intrinsic viscosity.
Unfortunately, improving one of these three properties—natural stretch ratio, crystallization rate, or fill injection rate—has normally resulted in detriment to one or both of the remaining properties. For example, compositions with suitably slow crystallization rates often require a higher intrinsic viscosity to maintain an adequate natural stretch ratio, which adversely impacts the fill time in injection molding.